In recent years, some environmental chemical substances have been reported to have estrogen-like activity, and for example, feminization of wildlife has been reported on some types of chemical substances (T. Colborn, D. Dumanoski and J. P. Myers, Our Stolen Future, 1996, Dutton, N.Y.). The activity of such chemical substances leads to a disturbance of hormone balance in various organisms including human and can cause disorders or diseases. Thus, the measurement of the estrogen-like activity of chemical substances has been attempted as part of a safety test for chemical substances.
Estrogen binds to the estrogen receptor in an estrogen target cell so that the receptor is activated to bind to chromosomal estrogen response element sequences. A transcription coupling factor, which recognizes the complex of the estrogen and the estrogen receptor, binds to the estrogen response element sequences to promote the expression of the genes downstream of the sequences. For the method of determining estrogen-like activity of chemical substances, therefore, there has been a need to develop an assay system for evaluating the ability of the chemical substances to regulate the estrogen receptor activity, and there has been a demand for an estrogen receptor gene which is applicable in such an assay system.